Authorities have told a Chinese heating company it will be fined for discharging dyed water into the Yellow River, turning a half-mile section bright red, a state-run newspaper said Tuesday.
Sunday's incident caused an uproar in Lanzhou, a city of 2 million and the capital of the western province of Gansu. Environmental officials rushed to take samples amid fears of a toxic spill.
It was the latest in a series of industrial accidents involving major rivers in China in the last year.
Lanzhou's environmental authority delivered the notice of the fine to the Lanzhou Tanjianzi No. 2 Heat Providing Station, according to the Beijing Youth Daily. It did not give details on the fine.
An employee answering the telephone at Tanjianzi said she was "not aware" a notice had been served but said the company was cooperating with environmental officials.
The woman, who refused to give her name because of the sensitivity of the issue, said hot water in boilers was dyed to prevent people from diverting it for their own use.
She said one of the boilers needed to be repaired and that about 400 gallons of dyed water in it was flushed out during the work and discharged into the river.
"The dyed material is not poisonous," the employee said. "We went to the Yellow River yesterday and found the water was no longer red."
Telephone calls to the environmental bureau were not answered Tuesday.
The Yellow River, which flows across northern China, is the source of drinking water to dozens of cities and tens of millions of people. It has thousands of chemical factories along its banks.
Protecting the environment has taken on new importance for the leadership following a November 2005 chemical spill in the Songhua River in northeastern China that forced the city of Harbin to shut down its water supply for days and sent toxins flowing into Russia.
China's major rivers, canals and lakes are badly polluted by industrial, agricultural and household pollution, and hundreds of millions of people live without adequate supplies of clean drinking water.
Protests have erupted throughout the country over complaints by farmers that uncontrolled discharges by factories are ruining crops and poisoning water supplies.